A multifaceted study is proposed to measure the viscous dissipation (viscosity) in red cells and red cell membrane. In addition, it is proposed to study the role of cell and membrane viscosity in governing the lateral diffusion of membrane proteins, the deformability of individual cells in capillary flow and the bulk rheological properties of concentrated cell suspensions. Measurement of the viscous properties of red cells and red cell membrane is accomplished by using micromanipulation and micropipet aspiration techniques to produce well-defined rates of deformation and flow of cells and membrane. These flows include (1) the continuous, axisymmetric flow of membrane from a spherical cell body to an elastic, cylindrical membrane strand or "tether", (2) the axisymmetric flow produced by the fusion of two cells (note that this particular experiment permits the nearly simultaneous measurement of viscosity and diffusivity), (3) the surface shear flow produced by the recovery of a red cell from an elongated shape to a biconcave disk and (4) the hemoglobin flow produced during the unfolding of a red cell after it is ejected from a small (3-4 Mum diameter) pipet. Continuous fluorescence photobleaching will be used to measure the diffusivity of fluorescent membrane particles as they diffuse from cell body to tether. The diffusivity subsequent to the fusion of two cells will be studied by following the motion of fluorescently tagged membrane proteins in the membrane of one of the cells as they diffuse across an equatorial line. Finally, the role of membrane and cellular viscosity in governing the deformability of cells will be determined by studying the flow of red cells through a single pore and by measuring the bulk rheological properties of microliter quantities of packed cell suspensions in a magneto-acoustic viscometer. From a fundamental viewpoint, membrane viscosity is a manifestation of the collective interaction of membrane molecules. Thus, studies of membrane viscosity give insight into membrane structure. From a practical viewpoint, viscous dissipation in the membrane surface influences its response to infectious disease, immunological reactions and hemolytic phenomena. In addition, the ability of red cell to respond to rapid shape changes as it moves through the circulation is influenced and controlled by the viscous nature of the membrane. All of these factors (membrane and cell viscosity, membrane diffusivity and cell deformability) will be studied.